The invention relates to a process and an apparatus for the joint oxidation and heat treatment of parts according to the preamble of the claims.
In the heat treatment of metal parts, generally, the gas atmosphere in the treatment compartment of a furnace is set in such a manner that a clean and bright metal surface is produced. Oxidation is undesirable in this case, since subsequent processes can be adversely affected by the oxide layers formed. However, there are special applications in which it is useful and expedient to produce an oxidic covering layer on the surface of the metal parts. The desired chemical composition and the thickness of these epitaxial oxidic covering layers make special requirements of the gas atmosphere in the treatment compartment of the furnace and the process procedure. By means of oxide layers produced in this defined manner, decorative surfaces can be produced which simultaneously act as corrosion protection, insulating layers or absorption layers.
The hardening and tempering or case hardening, carbonitriding etc. of solid parts is carried out in furnace systems, through the treatment compartment of which flow protective and/or reaction gases.
The parts are heated with the use of nitrogen in combination with reactive components or with endothermic atmosphere or the like to the particular austenitizing temperature, thermochemically treated and hardened by quenching in oil or water or liquid salt. In a subsequent second heat-treatment step, the required hardness is set by tempering the hardened parts. The tempering is generally performed at temperatures between 150xc2x0 C. and 570xc2x0 C. in an air or nitrogen atmosphere. Under these conditions, oxidation of the iron to magnetite (Fe3O4) can take place. The oxide layer does not have a sufficient layer thickness in this case to give an optically attractive decorative uniform surface.
Coating processes have therefore been used to produce a decorative uniform layer on the surfaces of the parts to be treated. Firmly adhering layers are applied to the surface by black-oxide coating, phosphatizing, enamelling and the like. These additional coating processes cause considerable process complexity and additional costs, with at times relatively non-uniform covering layers. Therefore, it has already been proposed to carry out hardening and blue-annealing in one process step (DE 43 33 940). This process was able to considerably decrease the coating costs, improve the process sequence, increase in the availability of the parts to be treated and decrease the use of chemicals, as also the emissions occurring in the process.
However, in the known process it would be desirable if the reproducibility could be increased with still more uniform oxide coat formation and the slight decarburization could be still further decreased.
The object underlying the invention is to provide a process for the joint oxidation and heat treatment of parts which is reproducible and leads to more uniform oxide layers of greater corrosion resistance.
This object is achieved according to the invention by the features specified in the claims.
Advantageous developments of the invention are specified in the subclaims.